Northwest Africa 5644 (NWA 5644)

(Northwest Africa)

Purchased: Nov 2008

Classification: Ungrouped achondrite

History: Purchased in Erfoud, Morocco, in November, 2008.

Physical characteristics: A light tan to rusty brown, angular single stone devoid of fusion crust, with a total mass of 200 g.

Petrography (Albert Jambon, Omar Boudouma and Dominique Badia, UPVI):  One polished section of about one cm² was studied by EMPA and SEM. The rock is medium grained, composed of light-brown pyroxene (0.2 to 1 mm) and interstitial mm-sized aggregates of plagioclase (0.01 to 0.05 mm) with lobate boundaries against pyroxene, sometimes with poikilitic pyroxene. Clusters of opaques up to several mm are conspicuous on both the sawn and altered surfaces, with poikilitic pyroxene, plagioclase and silica. Rims of Fe-rich olivine occur around pyroxene against ilmenite and spinel. Modes from BSE images (vol%) are: plagioclase 39, pyroxene 54. silica 2, fayalite 1, opaques 3. Contains minor troilite and rare pentlandite. Kamacite and taenite are partly oxidized.

Geochemistry: Pyroxene exhibits well-developed exsolution bands of augite (En₂₅Fs₃₂Wo₄₂) and pigeonite (En₃₁Fs₆₃Wo₆), but all intermediate compositions are present. The FeO/MnO ratio (73±7) is identical for augite and pigeonite, and distinctly higher than the HED range. Plagioclase is An_82-93. Minor phases are spinel (Us₆₄Ct₂₄), ilmenite, olivine (Fa₈₀), metal with up to 55% Ni and badly oxidized, pentlandite and silica. Shock is moderate and, with the exception of the oxidation of metal and sulfide, weathering effects are moderate.

Classification: In its texture, mineral mode and mineral compositions this rock is nearly identical to the achondrite NWA 011, which strongly suggests that these two stones are paired.

Specimens: 21 g and one polished section are on deposit at UPVI. A. Habibi holds the main mass.